1. Field of the Invention
This invention relates to novel microporous polyfluorocarbon membranes. More particularly, this invention relates to novel multilayer microporous polyfluorocarbon membranes, such as polytetrafluoroethylene membranes, which are asymmetric in pore size, i.e., which have an increasing gradation in average pore diameter from one surface of the membrane to the other. These membranes are particularly useful as microfilters, and especially as the filtration components of microfiltration cartridges, which can be used to remove fine particle size contaminants, including bacteria--particles having diameters as small as about 0.001 .mu.m--from fluids such as those encountered in the electronics and pharmaceutical industries. This invention also relates to a novel process for preparing these microporous asymmetric polyfluorocarbon membranes.
2. Description of the Prior Art
Porous polyfluorocarbon membranes or films, particularly ones fabricated in whole or in part from polytetrafluoroethylene, have been known for twenty-five years or more. Such membranes or films are usually prepared by either of two general methods:
1. Incorporating a particulate, solid, pore forming filler removable by leaching, heating, etc. into a mass of powdered fluorocarbon polymer; shaping the resulting mixture into a preform, calendering the thus-obtained preform to reduce its thickness and form it into a self-sustaining sheet or film, with or without doubling over or cross-lapping the sheet or film between passes through the calender rolls; subjecting the calendered sheet or film to heat to dry the sheet or film and sinter the fluorocarbon polymer particles, and removing the particulate filler; see, for example, U.S. Pat. Nos. 3,281,511 to Goldsmith; 3,518,332 to Sklarchuk et al; 3,556,161 to Roberts; 3,661,645 to Strier et al; 3,773,878 to Jahnke; 3,859,402 to Bintliff et al; 3,864,124 to Breton et al; 3,890,417 to Vallance; 4,003,818 to Juillard et al; 4,196,070 to Chao et al; 4,250,002; 4,297,196 and 4,498,961 to Lazarz et al; 4,292,146 and 4,342,636 to Chang et al and 4,380,521 to Moreno et al, and British Pat. Nos. 943,624 to Doulton & Co. Limited and 1,081,046 to Imperial Chemical Industries Limited.
2. Forming a porous fluorocarbon polymer membrane or film having nodes interconnected by fibrils using processes which involve, for example, stretching a preformed fluorocarbon polymer sheet, or extensively mixing and then biaxially calendering a putty-like mass containing fluorocarbon polymer; see, for example, U.S. Pat. Nos. 3,953,566 and 4,187,390 to Gore; 4,049,589 to Sakane; 4,089,758 to McAloon and 4,153,661 to Ree et al.
Commercially available porous polyfluorocarbon membranes or films made by the first of these methods, incorporation and subsequent removal of a particulate solid, are capable only of coarse filtration, i.e., removal from liquids of particulate matter having average particle diameters of not less than about 1 .mu.m, and in general also have inadequate strength, flux and dirt holding capacity to be useful in microfiltration applications. Hence, such membranes are used as coarse filters, or as separators or diaphragms in batteries and electrolytic cells.
Although porous polyfluorocarbon membranes or films having polymer nodes interconnected by fibrils have been used for microfiltration, such membranes are not without problems, some of which are attributable to their method of preparation. The membrane can itself be a major source of contamination because of its loosely bound fibrils and nodes. Also, it is difficult to control the processes by which such membranes are typically fabricated so that pore size will be correlated consistently with performance requirements.
There is thus a need for chemically and physiologically inert microporous polyfluorocarbon membranes whose pore structures render them particularly suitable for microfiltration applications.
It is an object of this invention to provide novel microporous polyfluorocarbon membranes.
It is also an object of this invention to provide novel microporous asymmetric polyfluorocarbon membranes useful as filtration means.
Another object of this invention is to provide novel microporous asymmetric polyfluorocarbon membranes, such as polytetrafluoroethylene membranes, whose unique pore structure, high porosity, high strength, high flux, low particle shedding and superior dirt holding capacity make them particularly useful as microfilters, and especially as the filtration components of microfiltration cartridges, which can be used to remove fine particle size contaminants from fluids such as those encountered in the electronics and pharmaceutical industries.
A further object of this invention is to provide a novel process for preparing these microporous polyfluorocarbon membranes.
These and other objects, as well as the nature, scope and utilization of the invention, will become readily apparent to those skilled in the art from the following description, the drawings and the appended claims.